1. Field of the Invention
The present invention relates to an apparatus for recording and/or reproducing information on an optical recording medium, for example a magneto-optical disk drive, a direct read after write drive (DRAW), a phase changing disk drive, CD-ROM or the like.
2. Related Art Statement
The disks for use in such an apparatus are provided with a protective layer (hereinafter, referred to as a cover glass) such as glass, resin (PC, PMMA or the like) formed on its side surface on which the reading light and the writing light are incident, in order to protect or guard the recording layer of the recording medium. This cover glass has different thicknesses in accordance with the types of the disk, even in the same DRAW disk. For example, the thicknesses of the disks are 1.2 mm for the magneto-optical disk, 1.2 mm and 0.6 mm for the phase changing disk, and 1.2 mm and 0.09 mm for the standard DRAW disk of 356 mm.
An objective lens for use in an optical pick-up of the information recording and/or reproducing apparatus has generally a numeral aperture NA of 0.45-0.6, and is designed by taking aberration caused by the above-discussed cover glass into consideration. On considering allowable level at the reading/writing property of the signal, in the case of the cover glass having a 1.2 mm thickness, the limit of the allowable level is in the order of .+-.0.05. If the allowable level exceeds this limit value, the reading/writing property of the signal becomes considerably deteriorated. If the reading/writing operation of the signal is performed by exchanging the disks having a different thickness of the cover glass, for example, the disk having 1.2 mm thickness of the cover glass and the disk having 20.6 mm thickness of the cover glass, therefore, it is difficult to perform this reading/writing operation of the signal with one information recording and/or reproducing apparatus. That is, two information recording and/or reproducing apparatus must be required.
In order to remove the above defect, for example, Japanese Patent Application Opened No. 241,095/93 provides the feature of inserting parallel plates between a light source and a collimator to compensate a spherical aberration caused by the difference in thickness of the cover glass of the disk.
However, the optical pick-up disclosed in the Japanese Patent Application Opened No. 241,095/93 has a problem as follows.
For example, a spherical aberration component .omega..sub.40 of a coefficient of a wave front aberration and a value .upsilon. of the wave front aberration, which are caused, for example, in case of passing the light through the cover glass having thickness t can be obtained from "Optics 14" (1985), pp. 219-221 and expressed as follows: EQU .omega..sub.40 =(t/8).multidot.{(n.sub.2 -1)/n.sub.3 }.multidot.NA.sup.4 (1) EQU .upsilon.=0.0745 .vertline..omega..sub.40.vertline. (2)
wherein the numeral aperture of the objective lens is NA, and the refractive index is n. Therefore, in the pick-up having NA=0.55 and n=1.57 and using the light of wavelength .lambda.=780 nm, if the disk having the cover glass of t=1.2 mm is exchanged to the disk having the cover glass of t=0.6 mm, the spherical aberration of .omega..sub.40 =0.00260, .upsilon.=0.0248 .lambda.rms is generated.
While in case of compensating the above-discussed spherical aberration by arranging the parallel plates for compensation between the light source and the collimator lens, if the numerical aperture at the light source side is NA'=0.25 and the refractive index is n =1.57, the thickness t' of the parallel plates is expressed as follows. EQU t'={n.sub.3 /(.sub.2 -1)}.multidot.(8/NA'.sup.4).multidot..omega..sub.40 =14.07 mm
Moreover, in order to insert such thick parallel plates between the light source and the collimator lens, the light source must be moved by a following distance 1: EQU 1=t' (1-1/n)=5.11 mm.
In such a way, the optical pick-up disclosed in the Japanese Patent Application Opened No. 241,095/93 requires a moving mechanism for inserting the compensating parallel plates and a mechanism for moving the light source in accordance with the insertion of the parallel plates, so that the construction of the apparatus becomes complicated and the optical system becomes large. In the reading/writing operation, moreover, the objective lens is moved with the following up of the eccentricity of the disk by a tracking servo, thereby causing non-coincidence of the light axis of the objective lens to the light axis of the collimator lens, resulting in a generation of comma aberration or astigmatism.